Within the field of computer technology, a growing need has arisen for the production of single page or mutliple page hard copy output from computers. The introduction of computers having extensive color graphics capabilities has led to a demand for a corresponding capability to produce color printed or plotted output sheets having both text and graphics features. Three major technologies have arisen to provide this color hard copy output capability.
The initial color plot capability was provided by the use of complicated pen plotting mechanisms, which create output by a process of repetitive line drawing using one of a selection of colored ink pens on a sheet of material, usually paper or film. Such output is constrained, by the mechanics of the system, to be a line plot. Shaded areas and color toning are not possible with such a system.
More recently a series of dot matrix color printers have been developed, utilizing an extension of known black and white dot matrix printing techniques, whereby a printhead containing a plurality of fine wires is traversed laterally across an output sheet. The wires are propelled forward in a controlled sequence as the printhead transitions across the sheet, impacting through an inked ribbon, forming characters from a series of dots created by the impact wire upon the output sheet. Since the text characters formed are formed from a regular sequence of dots within an overall matrix controlled by the systems software, this has come to be known as dot matrix printing. An extension to this technology provides a color output capability by providing an ink ribbon having multiple colors which is positioned under the ink printhead once for each color to be printed. Dot matrix printers are known to be relatively slow, high wear mechanisms that generate a great deal of noise. In addition, when color is involved, the necessity of making multiple passes for each line, one for each color, creates a time consuming process. Registration errors of the dot matrix printhead during each of the passes create areas having incompletely filled dots for a blurred appearance. Wear and aging of the ink ribbon produces a faded color appearance, which is quite noticeable.
More recently an alternate series of hard copy print output devices has been developed using what is known as ink jet technology. As with dot matrix printers, ink jet printers produce their output copy by transitioning a printhead laterally across a sheet of paper or similar material upon which the image is to be fixed. The ink jet plotter creates the image by emitting, in a controlled fashion, a jet of individual droplets of ink. These drops form the image upon the output sheet, much in the manner that a dot matrix printer forms the image. Ink jet plotters or printers have certain specific advantages. They are essentially noiseless. They can be transitioned at a much higher rate of speed, which means a higher print speed, because there is no necessity for physical contact of the printhead with the output sheet and because the number of moving parts within the plotter or printer has been significantly decreased from that of a dot matrix unit. Because the plotters print by means of a multiple droplet spreading effect, droplets can be merged or oversprayed so as to produce an increased shading capability; in color printing an increased range of colors can be shown. Additionally, the spreading of drops produces a more solid image than has been possible with the previous two technologies.
With ink jet plotters, color capability is provided by one of two technologies. Both require independent chambers for each of the colors of inks provided. An airtight ink cartridge is used to feed ink to channels aligned in rows on the printhead. Each channel has a separate orifice from which drops of ink are selectively extruded and deposited upon the paper. A slight negative pressure within each channel keeps the ink from inadvertently escaping through the orifice, and also forms a slightly concave meniscus at the orifice serving to keep the orifice clean. Such color ink units primarily use the three primary subtractive colors: cyan, yellow and magenta. These inks can produce, in general, up to one hundred and twenty-five shades or color combinations.
The first technology or Drop on Demand provides the drops of ink for impact upon the paper by means of a pulsed pumping or jet mechanism within each chamber, usually in the form of a piezoelectric pump which creates small pressure pulses within the orifice causing the ejection and formation of a flying drop which crosses the space between the printhead and the printed sheet, impacting upon the printed sheet. The overall formation of the images is under the control of software or computer control which controls the individual formation of drops as required to create the desired image.
The dot resolution of the printhead is dependent upon the spacing of the individual color orifices; the closer and smaller the orifices, the greater the resolution and the more detailed the plot than can be produced. Since this technology requires separate ink channels for each colored ink, the fact that there are at least three channels required to produce the colors tends to degrade the overall resolution that can otherwise be achieved.
An alternative form of printhead, the Continuous Stream printhead, is considered faster than Drop on Demand printing and produces somewhat higher quality print and graphics. This method uses a single channel upon the printhead for each color and an oscillating pump which produces a continuous stream of drops. The drops of ink are electrically charged and are then directed to the position on the paper or the printed sheet by means of deflection electrodes. Where no print is desired, the drops are deflected into an ink capturing mechanism or gutter from whence the ink is either recycled or disposed of. The resolution of such a head is not degraded by the use of multiple ink colors; however, each separate color must of necessity have its own individual focusing, deflection, and gutter system in order for the Continuous Stream head to function correctly. Further, a Continuous Stream system wastes a great deal of ink. The consumption of ink is much higher in a Continuous Stream than in a Drop on Demand ink jet head.